1. Field of the Invention
The present invention relates to a narrow-angle directional microphone that accommodates in a cylindrical microphone case an acoustic tube, in the circumferential wall of which an opening is formed to be covered with an acoustic resistor such as non-woven fabric and to the rear end of which a microphone unit is attached, and more particularly, to a narrow-angle directional microphone that prevents abnormal noise from occurring due to the vibration from the acoustic tube and the acoustic resistor.
2. Description of Related Art
A narrow-angle directional microphone has been known which uses an elongated acoustic tube. In the narrow-angle directional microphone, a microphone unit attached to the rear end of the acoustic tube detects a sound wave input through an opening of the front end of the acoustic tube and converts the sound wave into an audio signal.
In an example of the narrow-angle directional microphone using the acoustic tube according to the related art, as illustrated in FIG. 4, an acoustic tube 50 made from a metallic tube is used, openings 51 having a slit shape are provided in the circumferential wall of the acoustic tube 50 along the central axis direction, and an acoustic resistor 52 made of e.g., a synthetic resin thin film or non-woven fabric is attached to the circumferential wall of the acoustic tube 50 to cover the openings 51.
In the narrow-angle directional microphone having the above-mentioned configuration, among the sound waves input from an opening 53 provided at the front end of the acoustic tube 50, a sound wave which turns around in the circumferential direction interferes with a sound wave which is transmitted from the openings 51 provided in the circumferential wall of the acoustic tube 50 through the acoustic resistor 52. In this way, narrow-angle directionality is realized.
JP 2010-245994 A discloses the above-mentioned narrow-angle directional microphone using the elongated acoustic tube.
However, there is a problem that when the openings 51 provided in the circumferential wall are especially large, since it is easy for the acoustic resistor 52 to make vibration, the acoustic resistor 52 makes vibration due to a sound wave with a large amplitude, unevenness is formed in a directional frequency response, and thus abnormal noise occurs.
In addition, when mechanical vibration is applied to the acoustic tube 50 and resonance occurs at a specific frequency, then abnormal noise occurs.
Further, as illustrated in a cross-sectional view in the radial direction in FIG. 5, the acoustic tube 50 is accommodated in a cylindrical microphone case 60 and used for protective purpose, but when mechanical force (vibration or impact) is applied to the microphone case 60 and the acoustic tube 50 is relatively displaced, then abnormal noise occurs.